The present invention relates to a novel thickener composition and a method for the preparation thereof. More particularly, the present invention relates to a thickener composition which is an aqueous solution containing a high molecular weight polysaccharide derivative having remarkably improved thermal stability dissolved therein as well as a method for the preparation of such an aqueous thickener composition by the reaction of a high molecular weight polysaccharide having a specific superhelix structure with a low molecular weight aliphatic aldehyde in an aqueous medium in the presence of an inorganic salt.
Along with the trend of exhaustion of the petroleum resources in recent years, development of the technology is now under way of extensive studies for the secondary and tertiary recovery of petroleum.
One of the typical processes within such a technology is a method for the enhanced recovery of petroleum generally called the "enhanced oil recovery" while the efficiency of this process largely depends on the viscosity of the polymeric thickener composition used therein.
Generally, the polymeric thickener composition used in the process of enhanced oil recovery should have several characteristic properties including, for example, a high viscosity of an aqueous solution thereof even in a relatively low concentration, stability against high shearing forces, insusceptibility to the influences of the variations in the pH value or presence of polyvalent metallic ions, non-Newtonian flow behavior of the solution, durability under a high-temperature and the like.
Recently, several polymeric materials are proposed as one suitable for use as the principal ingredient in such a thickener composition including high molecular weight polysaccharides biologically synthesized by certain microorganisms or so-called biopolymers such as xanthan gum, PS-7 and the like. These biopolymers are not quite satisfactory in respect of the insufficient thermal stability at elevated temperatures for practical use.
For example, the viscosity of an aqueous solution of xanthan gum at 60.degree. C. is only about one tenth of the value at 20.degree. C. This is mainly due to the fact that the elongated fine superhelix structure as the characteristic secondary structure of the polysaccharide at room temperature is denatured when the temperature is risen over about 60.degree. C. so that the desired effect for thickening can no longer be exhibited at higher temperatures. Moreover, the viscocities of aqueous solutions of these high molecular weight polysaccharides vary remarkably at a pH of about 7 as a result of a reversible denaturation-renaturation of the fine structure of the polysaccharide due to the possible dissociation of the electrolyte groups bonded to each of the structural units.